Phase coherence length ($L_{\varphi }$)

The characteristic distance over which phase-breaking processes suppress electron-path interference. For diffusion, $L_{\varphi }\sim \sqrt{D{\tau }_{\varphi }}$; ballistic phase propagation instead gives a scale $v{\tau }_{\varphi }$. This differs from the elastic mean free path, since elastic scattering can change momentum while retaining coherence.